1. Field of the Invention
The present invention relates to traffic management in a communications network and, in particular, to filtering of traffic in said network.
2. Prior Art
The use of filter systems to manage the flow of traffic in a communications network is well known in the prior art. In a conventional filtering system, selected characteristics of an unknown item are subjected to rules which are known to classify items. If the unknown item fits the criteria of a rule, then the action associated with that rule is taken. Rules may have priority so that an item which passes several rules is actually subjected to a unique action, namely that of the highest priority rule it fits. In some systems rules may be of several types, in which case the rule to apply would be the rule of each type which fits and has highest priority.
One straightforward or brute force way of filtering an item is to subject the item to all rules sequentially, in order of priority. Compared to the method disclosed herein, the sequential method is simple but slow. This brute force approach is acceptable only if the number of rules is small or speed is not essential. In a communications system where the universe could have tens or hundreds of rules and time is very critical, this brute force approach is not acceptable.
Another brute force way of filtering an item is to subject the item to all rules in parallel, then accumulate the results and find the rule with highest priority passed by the item. Compared to the method disclosed herein, the parallel method is simple and fast, but also expensive and inefficient. A specialized chip capable of performing parallel compares would be required to carry out the comparisons. Such a chip might not perform any other function. Consequently, the cost cannot be leveraged to a multifunction chip.
The prior art has improved the brute force methods by utilizing tree structures in the filtering process. U.S. Pat. No. 5,463,777 describes a tree structure in which the key segments of multiple bits (such as 32 bit segments) from the header of a frame are compared with chosen values at tree nodes. Results of the searches on segments are ANDed to produce a final result.
U.S. Pat. No. 5,574,910 also makes use of fixed length keys (such as 32 bits) and, in the case of a plurality of segments, ANDs results to reach a final decision.
U.S. Pat. No. 5,546,390 also teaches the advantages of choosing certain bits in building a search tree called a radix tree-type decision process. This method examines two or more bits at each tree branch. In fact, U.S. Pat. No. 5,546,390 distinguishes itself from its prior art in part by selecting two or more bits for each node in its decision tree nodes. Unfortunately, as mentioned therein, allowing 4, 8, or more branches at each node can result in wasted space for unused links. Those skilled in the art will recognize that it is not always feasible to find such combinations of bits in real rule sets.
U.S. Pat. No. 5,813,001 describes another search technique in which groups of bits from a search object are matched against groups of bits from entries in a knowledge base system. It is believed that this approach requires complex circuities or long processing time in order to perform the required group processing.
U.S. Pat. No. 5,136,580 uses a Programmable Logic Array (PLA) and a Content Addressable Memory (CAM) to recognize if a Destination Address (Recipient) and Source Address (Sender) are on the same LAN. If so, the frame is filtered and not forwarded to another LAN. Hence this system provides a limited filtering function.
U.S. Pat. Nos. 5,541,911 and 5,790,554 refer to filters in a general way, but appear not to disclose any specific structures or mechanism for filtering.
Even though the prior art patents may work satisfactorily for their intended purposes, none of the patents gives a mechanism for selection of bits to use at tree nodes.